1. Field of the Invention
The present invention relates to a nano-electrospray nebulizer for use as an ion source for a mass spectrometer. More particularly, it relates to a nebulizer that facilitates alignment with the mass spectrometer target, reduces sample and nebulizer gas leakage, and is easy to assemble and disassemble so as to permit quick electrospray needle replacement for elimination of clogs, operation over a wide range of sample flow rates, including extremely low sample flow rates without loss of accuracy, and performance of both static and dynamic analysis.
Various types of mass spectrometers are available in the market. These include quadrupole, magnetic sector, Fourier transform ion cyclotron resonance and other time-of flight devices as well as hybrid combinations of mass spectrometers, (BEQQ, Q-Tof, TOF-TOF, Ion Trap-Fourier Transform, etc.).
Those mass spectrometers operate by causing charged molecules (ions) of the sample to be analyzed as a function of their mass to charge ratio. The separated ions are detected electronically. The detected quasi-molecular ions are detected as an ion current which is directly correlated to the different elements or empirical formulas of the compounds that make up the sample. Accordingly, mass spectrometers of the type here under consideration require an ion source that includes a means of ionizing the molecules of the sample to be analyzed while in solution phase.
2. Description of Prior Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Electrospray and nanospray techniques are conventionally utilized for converting an initially neutral sample into an ionized species in the gas phase. Devices utilizing those techniques are therefore commonly used as sources of ions for mass spectrometers.
Those ion sources consist of a needle assembly, which includes a very thin, hollow, inner metal or glass tube. The inlet side of the inner tube is connected through a housing to receive the sample to be analyzed. A liquid chromatographic column or an infusion pump may be used to supply the sample to the housing. The sample to be analyzed is normally supplied to the housing as a liquid formed of a solid in solution.
The needle assembly also includes a hollow, metal outer tube having an inner diameter that is slightly larger than the outer diameter of the inner glass tube. The inner tube is received within the outer tube such that the outer tube surrounds, but is spaced a short distance from, the inner tube, defining a space around the inner tube through which a nebulizer gas is pumped.
The outlet side of the needle assembly terminates in a chamber within the mass spectrometer. The walls of the outer metal tube electronically charged relative to the source chamber. The liquid sample is converted by the needle assembly into an electrostatically charged aerosol spray that is discharged from the needle assembly outlet into the chamber. The solvent in the liquid sample is evaporated. The analyte and solvent ions in the aerosol spray are moved by a vacuum pump from the chamber through a target having a conical aperture and are then drawn into the analyzer.
In practice, the outlet side of the needle assembly must be very precisely aligned with the target in order to deliver sufficient amounts of sample to the analyzer for analysis. In many designs, the needle assembly provides an ion spray that is aimed in a direction orthogonal to the axis of the target aperture. Accurate alignment of nebulizers with that structure, such as the electrospray ionizer supplied by Micromass UK Limited of Floats Road, Wythenshawe, M23 9LZ, U.K. with its Q-TOF 2 hybrid quadrupole time-of-flight mass spectrometer, involves painstaking and time-consuming adjustments to achieve the required alignment.
Aside from the alignment problems, sample and nebulizer gas leaks from the needle assembly commonly occur in the Micromass ion source. This is due to the structure of the housing that functions to connect the needle assembly inlet with the sample source and the nebulizer gas source.
Clogs in the needle assembly tend to be a common occurrence in ion sources utilizing the tube within a tube structure because of the very small inner diameter of the inner glass tube. In order to avoid having to replace the entire tube when a clog occurs, requiring removal of the nebulizer housing from its platform and the subsequent time consuming realignment of the needle assembly with the target, a “quick fix” is sometimes attempted in the Micromass ionizer by cutting off the end of the clogged inner glass tube from the remainder of the needle assembly. Unfortunately, that procedure often does not work well because cutting off the clogged portion of the inner tube frequently results in an uneven or jagged edge (on a microscopic scale) that alters the direction of the spray.
On the other hand, aside from the alignment problems noted above, replacement of the clogged needle assembly is also disadvantageous because the glass inner tube is extremely thin and fragile. It is very difficult to handle and breaks easily. Further, the ends of the inner glass tube are very sharp and the tube must be very handled carefully to avoid injuring the personnel setting up the apparatus.
For all of those reasons, set up of the analysis equipment is labor intensive and very time consuming. We have found that it sometimes requires several hours to set up a single analysis with the Micromass equipment.
Some conventional nebulizers designed for use with mass spectrometers, including the one supplied by Micromass for the above mentioned analyzer, have what is known as a “zero dead volume” design that is suppose to eliminate any gap or space between the sample supply conduit and the inlet of the inner tube of the needle assembly. Under low flow conditions, such a gap is undesirable because a “dead volume” within the housing results in a substantial time delay between the time the sample is introduced into the housing and the time that the ions are formed.
The Micromass needle assembly housing incorporates a blind recessed fitting to interface the inlet end of inner glass tube to the zero dead volume union. However, this structure increases the difficulty of the alignment of the assembly parts. It also greatly increases the risk that a leaky connection will go unnoticed. The nebulizer of the present invention utilizes a structure at this interface that minimizes the alignment and leakage problems.
Conventional nebulizers, including the one from Micromass, tend not to operate well at very low liquid sample flow rates. Clogs in the needle assembly often result from such low flow rates, requiring removal of the nebulizer housing from its platform, cutting off of the clogged portion of the inner tube, and time consuming reassembly and realignment of the apparatus.
The nebulizer of the present invention allows for rapid changing of the needle assembly to a larger diameter inner glass tube with reduced chance for clogging and hence can be used over a wider range of flow rates. Since the diameter of the inner tube is proportional to its capacity and maximum flow rate, quick changing of the tube permits our apparatus to accommodate flow rates from as low as 20 nanoliters/minute to as much as 200 microliters/minute, a dynamic range of 1000.
Further, we are able to interface our source to a variety of different supplies, from a 75 micron inner diameter liquid chromatographic (LC) column (having an optimum flow rate of 150 nanoliters/minute) to a 300 micron inner diameter LC column (having an optimum flow rate of 2 microliters/minute) and a 2 millimeter inner diameter LC column (having an optimum flow rate of 200 microliters/minute). This provides greatly enhanced versatility.
Another feature of our ionizer structure is that it can reproducibly be installed “off the shelf” in a much shorter time than the Micromass design because the parts are self-aligning and easier to assemble. In part, this is due to the use of a “snap-in snap-out” spring clip mechanism that is keyed to reproducibly align the needle assembly with the previous micrometer settings, along the X, Y and Z directions. The Micromass equipment has no comparable structure and can only be adjusted in two directions.
Our design also allows the flexibility of screwing it directly into a capillary column sample supply. This reduces the unwanted dead volume in the housing and hence the amount of time it takes to begin the analysis. For example, a 500 nanoliter dead volume will take ten minutes to fill at 50 nanoliters/minute.
The present invention has a mounting platform for the needle assembly housing that is aimed at a 45 degree angle relative to the axis of the target. That configuration allows for a greater cross-sectional area of the ion spray to enter the target aperture, as compared to the orthogonal approach utilized by Micromass.
Different nebulizers with different size needles are required for static and for dynamic analysis. The Micromass equipment requires time consuming reassembly and realignment when changing between types of analysis. That problem is largely eliminated with our design because of the ease by which the needle assembly can be replaced.
It is, therefore, a prime object of the present invention to provide a nano-electrospray nebulizer for use with a mass spectrometer that facilitates assembly and alignment of the needle assembly.
It is another object of the present invention to provide a nano-electrospray nebulizer that facilitates quick and easy removal and/or replacement of the inner tube of the needle assembly to eliminate clogs, and to permit use of the nebulizer with samples having a wide range of sample flow rates and for both static and dynamic analysis.
It is another object of the present invention to provide a nano-electrospray nebulizer structure that greatly reduces sample and nebulizer gas leaks.
It is another object of the present invention to provide a nano-electrospray nebulizer structure that reduces the dead volume within the needle assembly housing and hence speeds up the analysis.
It is another object of the present invention to provide a nano-electrospray nebulizer structure that is adjustable in three directions to facilitate target alignment.
It is another object of the present invention to provide a nano-electrospray nebulizer structure that includes a spring clip mechanism permitting the needle assembly housing to be quickly and easily removed from and mounted on the mounting platform without disturbing the previous alignment setting.
It is another object of the present invention to provide a nano-electrospray nebulizer which is capable of operating at extremely low liquid sample flow rates, thereby requiring less sample material, without loss of sensitivity or accuracy, or significant reduction in signal-to-noise ratio.
It is another object of the present invention to provide a nano-electrospray nebulizer structure in which the needle assembly housing mounting platform is oriented at a 45 degree angle relative to the target axis to increase the amount of ions entering the target.